1. Field of the Invention
This invention relates to lithographic printing form precursors. The invention relates further to their manufacture and use. More particularly, this invention relates to printing form precursors comprising a thermally imageable coating on a substrate, wherein the coating comprises a composition including a hydroxyl group-containing polymer.
2. Background Information
The art of lithographic printing is based on the immiscibility of ink, generally an oily formulation, and water, wherein in the traditional method the ink is preferentially retained by the image or pattern area and the water or fountain solution is preferentially retained by the non-image or non-pattern area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water whilst the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A generally used type of lithographic printing form precursor (by which we mean a coated printing form prior to exposure and development) has a radiation sensitive coating applied to an aluminum substrate. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to radiation of a suitable wavelength hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. On the other hand positive working lithographic printing form precursors have a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. In both cases only the image area on the printing form itself is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing form precursor with a vacuum to ensure good contact. The printing form precursor is then exposed to a radiation source; conventionally this has been a UV radiation source. In the case where a positive form precursor is used, the area of the film that corresponds to the image in the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed on development.
Many positive working systems rely on the inhibition of the inherent solubility of phenolic resins, in suitable developers. Traditionally this has been achieved through the use of diazide moieties, especially naphthoquinone diazide (NQD) moieties, to provide compositions which only following exposure to UV radiation are soluble in the developer.
As demands on the performance of UV-sensitive positive working coatings have increased so NQD technology has become limiting. In addition, digital and laser imaging technology is making new demands on coatings for lithographic printing.
It is known from GB 1245924 that the solubility of phenolic resins in lithographic developers may be increased by the application of heat. The heat may be delivered by infra-red radiation, assisted by radiation absorbing components such as carbon black or Milori Blue (C.I. Pigment Blue 27). However the developer resistance of the non-exposed areas to commercial developers is low, and the solubility differential is low compared to the commercial UV sensitive compositions containing NQD moieties.
We have devised new positive working heat sensitive systems to meet the new demands. Our new systems and methods are the subject of our patents and patent applications including EP 825927B, WO 99/01795, WO 99/01796, WO 99/21725 and WO 99/11458. We have observed that in our new systems there may be an alteration in their sensitivity over time, after the heat sensitive composition has been applied to a substrate and dried, such effect being the result of reduced developer solubility of the unexposed compositions with time prior to exposure. Thus when we refer to “sensitivity” herein we are considering this in the context of the entire process of exposure and development. We are not referring to the matter of how the areas of the composition which are exposed react to that exposure. Sometimes this “sensitivity” is called “operating speed” in the art.
In order to overcome these problems we have devised a process which improves the systems mentioned above, such that a consistent and stable material can be supplied to an end user. This process is the subject of our patent application WO 99/21715.
WO 99/21715 discloses a method of manufacturing lithographic printing forms which includes a step of heat treating the forms, after the application and drying of the coating on the substrate, for an extended time period at 40-90° C. It is found that such heat treatment improves later exposure processes, in particular by rendering the sensitivity of the coating less variable, over time.
However, although this method is useful for providing stable and consistent lithographic printing forms, there are penalties in increased cost and production time.
We have now devised a system which produces stable and consistent lithographic printing forms without a requirement for the heat treatment step disclosed in WO 99/21715, and so offers the prospect of reduced production costs.
The compositions applied to the lithographic printing form precursors of EP 825927B, WO 98/31544, WO 99/01795, WO 99/01796, WO 99/21725 and the heat treated stabilised printing forms of WO 99/21715, have all previously been applied at coating weights of at least 1.2 gm−2, and often considerably more.
It has been found that printing form precursors which carry certain thermally imagable compositions at low film weights do not need a heat treatment step of the type described in WO 99/21715 as part of their manufacture in order to render the sensitivity of the compositions less variable over time. It has also been found that precursors having low weights of the compositions have good resistance to handling and transportation scratch damage, and therefore the necessity to add scratch resistance additives, which may increase cost and diminish performance, is reduced.
In our patent application WO 99/11458, there are disclosed examples of phenolic compositions which are applied to substrates to form lithographic printing form precursors. In the general passages a printing form precursor is described as having an imaging layer of thickness preferably between about 0.5 and about 3 micrometers. In some of the examples coatings were applied to give a final polymeric coating weight stated to be between 1.0 and 1.5 gm−2.
In our patent application WO 98/42507 there are described examples of phenolic resin compositions which are applied to substrates to form lithographic printing form precursors. In the general passages a printing form precursor is described as having an imaging layer of thickness, after drying, typically in the range from 0.5 to 2 m, and preferably from 1 to 1.5 m. In all of the examples the formulation was applied to give a dry coating weight of about 1.5 gm−2.
In EP-A-894622 there are disclosed printing plate precursors having a polymeric coating which comprises a resin with phenolic hydroxyl groups and a copolymer comprising, for example, a sulfonamido group or an acrylate group. In the general passages the coated solids amount after drying is said to desirably be in the range 0.5 to 5.0 gm−2. It is stated that as the coated amount decreases, the characteristics of the photosensitive layer become poor, although apparent sensitivity increases. In the examples in EP-A-894622 the coating amount of the polymeric coating, after drying, is 1.8 gm−2.
In the related specifications EP-A-901902, EP-A-909657 and EP-A-914964 there is the same general reference to a coating weight of 0.5 to 5.0 gm−2 and, in the examples, the coating weights are 1.4, 1.5, 1.8 and 2.0 gm−2.
The foregoing specifications provide no encouragement to look at low coating weights. They in no way enable the reader to conclude or infer that use of a low weight of a coating may be beneficial in rendering the sensitivity of the coating less variable over time and/or in improving its mechanical robustness.